Such a circuit known from the prior art is illustrated in FIG. 1, in which the input voltage is given the reference Ue, the output voltage the reference Ua, the switch the reference S1, the control input the reference St, the first diode the reference D1 and the inductance the reference LT. In this case, the input voltage Ue is provided by an input capacitor Ce. The output voltage Ua can be tapped off at an output capacitor Ca. A shunt resistor Rshe is used for current measurement purposes.
The problem on which the invention is based will be described in the text which follows with reference to the step-down controller circuit illustrated in FIG. 1: once the switch S1 has been closed, current flows in the circuit S1, LT, Ca, Rshe, Ce. Owing to this current flow, the inductance LT is magnetized and the output capacitor Ca is charged. If the switch S1 is then opened, the inductance LT attempts to maintain the current flow and drives a current in the circuit LT, Ca, Rshe, D1. Owing to parasitic line inductances in the diode path of the diode D1 which are combined here in the inductance Lpar, initially current flow is prevented via the diode path of the diode D1, however. The voltage UDr. present across the diode D1 increases, which becomes apparent in undesirable voltage peaks and in the consequence of these voltage peaks in terms of electromagnetic interference (EMIR). With this known step-down controller circuit, this is counteracted by a trapezoidal capacitor CTri being provided which takes over the initial current flow when the switch S1 is turned off. Although at some point the trapezoidal capacitor CTri is then charged, in the meantime the parasitic inductance Lpar is magnetized such that the current flow can be taken over by the diode path of the diode D1. A further disadvantage results by virtue of the fact that the switch S1 needs to have very large dimensions as regards its power loss. This results from a limited switching time, i.e. when S1 is turned off, a voltage USt. is present across the switch S1 while, at the same time, a current is flowing through it. The product of these two variables reproduces the power loss converted in the switch S1. As a result of this power loss, the switch S1 needs to be given excess dimensions, i.e. needs to have markedly larger dimensions than would actually be necessary as regards the load to be driven by it at the output terminals, preferably a lamp. Moreover, the energy stored in the trapezoidal capacitor CTri is not used expediently but is converted into heat in the switch S1. As is obvious to those skilled in the art, charging of the trapezoidal capacitor CTri directly via the switch S1 is not regarded favorably as a result of the high load on the switch and is therefore likewise undesirable.